1. Field of the Invention
The present invention relates to a controller of an injection molding machine.
2. Description of the Related Art
In a metering step of an injection molding machine, metering is performed by changing the retreat speed of a screw so that a set back pressure is obtained while rotating the screw at a set rotational speed, and the retreat speed of the screw is decreased so that the screw stops at a set metering completion position immediately before the set metering completion position to stop the screw at the set metering completion position. When decreasing the screw retreat speed but not changing the rotational speed of the screw, a resin pressure (back pressure) may rise. When the resin pressure rises, melted resin densities are changed, a fluctuation occurs, and the weight of a molded product fluctuates.
Therefore, a metering control method is publicly known (for example, refer to Japanese Patent Application Laid-Open No. 11-129296) in which the fluctuation of the density of a melted resin is decreased by performing feedback control of a resin pressure even at the end of a metering step in a state or close to a state of stop of screw rotation and stopping the pressure feedback control when retreat of a screw stops or approaches to a stop state of the retreat.
Moreover, a method for uniforming metering by preparing a plurality of combinations of back pressure commands and screw rotational speed commands, controlling back pressures and screw rotational speeds in association with each other base on the combinations, and controlling a screw rotational speed and a screw retreat speed so as to decrease them to zero is also known (for example, refer to Japanese Examined Patent Publication No. 1-26857).
Moreover, a method for obtaining a stable metered resin amount by sequentially decreasing a screw rotational speed before stop of metering on the basis of a screw retreat position is also known (for example, refer to Japanese Examined Patent Publication No. 64-6931).
Furthermore, a method for setting a pressure state in which a check valve is easily closed during injection by causing the screw to retreat without rotating the screw (that is, performing sack back) after metering is completed, lowering the pressure of a melted resin at the front end of a cylinder, and thereby preventing the melted resin from trailing down from a nozzle (drooling phenomenon) is generally used. Furthermore, a method for improving the sealing at the time of injection by reversing a screw after metering is completed and closing a check valve is also known (for example, refer to Japanese Patent No. 3118188 and Japanese Examined Patent Publication No. 45-2988).
In the case of the above-described conventional metering methods, an excessive long time is required in the deceleration section of a screw retreat speed nearby a metering completion position and thereby a metering time tends to be lengthened. Moreover, each of the conventional methods is a method for stabilizing a resin state in metering steps up to completion of metering.
As a method for stabilizing a resin state after completion of metering, the above sack back operation or a method for improving the sealing at the time of injection by reversing a screw after completion of metering and closing a check valve is a stabilization processing after completion of metering, which is performed between a metering step and an injection step. Thus, there is a disadvantage that a molding cycle is lengthened by the time required for performing such processing.